Mobile communication applications have used a variety of frequency bands, and it has been desirable to handle a plurality of applications in radios. Some applications, typified by CDMA, require control of output power, particularly lower power consumption, over a wide dynamic range. Further, smaller radios reduced in weight have been also desired.
A conventional power amplifier will be discussed below in accordance with accompanying drawings.
FIG. 7 is a block diagram showing a conventional power amplifier 700. In FIG. 7, reference numeral 701 denotes a first input terminal, reference numeral 702 denotes a first input-side matching circuit, reference numeral 703 denotes a first input-side DC bias supply circuit, reference numeral 704 denotes a first transistor, reference numeral 705 denotes a first output-side DC bias supply circuit, reference numeral 706 denotes a first output-side matching circuit, reference numeral 707 denotes a first output terminal, reference numeral 708 denotes a second input terminal, reference numeral 709 denotes a second input-side matching circuit, reference numeral 710 denotes a second input-side DC bias supply circuit, reference numeral 711 denotes a second transistor, reference numeral 712 denotes a second output-side DC bias supply circuit, reference numeral 713 denotes a second output-side matching circuit, and reference numeral 714 denotes a second output terminal.
The conventional power amplifier configured in this way performs operations on signals of two kinds of frequencies. The operation is performed as follows. In the case of an operation at a first frequency, a signal inputted to the first input terminal 701 is amplified by the first transistor 704 and is outputted to the first output terminal 707. In the case of an operation at a second frequency, a signal inputted to the second input terminal 708 is amplified by the second transistor 711 and is outputted to the second output terminal 714.
Moreover, for the signal inputs of the two kinds of frequencies, even when a signal with low output is obtained, the signal is passed through the first transistor 704 or the second transistor 712 and an amplifying operation is performed.
However, in such a configuration, there is a problem that the power amplifier is completely independent at each of the frequencies, and the number of components (not shown) between the power amplifier and an antenna is required for two systems, resulting in a large radio.
Furthermore, there is another problem that in use for a multistage amplifier and so on, since a power amplifier is generally adjusted so as to have the highest efficiency at the maximum output, even when output is reduced and a signal of low power is necessary, the power amplifier is operated. Thus, efficiency is degraded overall.